The present invention relates to a liquid crystal display device and to a method of driving the same; and, more particularly, the invention relates to a technique applicable to a driving method which applies a gray scale voltage to video signal lines within a vertical retrace interval.
Active matrix liquid crystal display devices, which have active elements (for example, thin film transistors) for individual pixels and in which the active elements are driven in a switching manner, are widely used as display devices for notebook types of personal computers (hereinafter referred to simply as personal computer(s)).
A TFT type of liquid crystal display module represents an example of one kind of active matrix liquid crystal display device. The TFT type of liquid crystal display module includes a TFT (Thin Film Transistor) type of liquid crystal display panel (TFT-LCD), drain drivers disposed on a longer side of the liquid crystal display panel, and gate drivers and an interface part, each of which is disposed on a shorter side of the liquid crystal display panel. In general, the drain drivers are driven on the basis of driving signals received from a display control device (or timing controller) provided in the interface part.
In the above-described type of liquid crystal display module, the interval from the completion of line scanning in the n-th frame until the start of line scanning in the next (n+1)-th frame is called a vertical retrace interval, and the line scanning period in each frame is called a display period.
One type of liquid crystal display module is constructed to output a gray scale voltage for displaying white or black from its drain drivers to its drain signal lines at intervals of one line scanning period within the vertical retrace interval, so that voltages written in its pixels are prevented from being varied, which causes lateral stripes on the display screen, owing to leakage currents from the thin film transistors of the pixels within the vertical retrace interval. Namely, in such a liquid crystal display module, even within the vertical retrace interval, a driving signal is transmitted from a display control device provided in its interface part to the drain drivers to drive the drain drivers.
However, if synchronizing signals inputted from the outside (for example, a computer host) vary and the vertical retrace interval varies, contention occurs between a driving signal transmitted from the display control circuit to the drain drivers within the vertical retrace interval and a driving signal transmitted from the display control circuit to the drain drivers within the display period of the next frame after the completion of the vertical retrace interval. This leads to the problem that the drain drivers malfunction, and, in the worst case, the drain drivers are destroyed.
The present invention has been made to solve the afore-mentioned problem, and it provides a technique by which, even when the vertical retrace interval varies, in a liquid crystal display device and a driving method thereof, it is possible to prevent contention from occurring between a driving signal transmitted from a display control circuit to a driving circuit within the vertical retrace interval and a driving signal transmitted from the display control circuit to the driving circuit within the display period of the next frame after the completion of the vertical retrace interval.
The invention also provides a technique by which, in the liquid crystal display device and the driving method thereof, it is possible to prevent the voltages written in pixels from being varied, so as to prevent lateral stripes from being generated on the display screen, thereby improving the display quality of the display screen.
The above and other novel features of the invention will become apparent from the following description of the invention when taken in conjunction with the accompanying drawings.